This invention relates to an air-fuel ratio feedback control method for internal combustion engines, and more particularly to a method of this kind which is applied when the engine is operating in a high load or high speed condition.
An air-fuel ratio feedback control method is conventionally known, in which the amount of fuel supplied to the engine is increased by the use of an incremental proportional term when the output from an exhaust gas concentration sensor arranged in the exhaust system of the engine changes from a rich side to a lean side with respect to a predetermined reference value, whereas the amount of fuel is decreased by the use of a decremental proportional term when the output from the sensor changes from the lean side to the rich side.
An improvement in such a method has been proposed by the present assignee in Japanese Provisional Patent Publication (Kokai) No. 63-246432 in which when the engine has continued to operate under a steady heavy load condition over a predetermined time period, the incremental proportional term is set to a larger value in order to reduce the amount of NOx emitted under such a steady heavy load operating condition of the engine such as high speed cruising.
According to the above manner proposed by the present assignee of further enriching the air-fuel ratio, the amount of change in the air-fuel ratio in the enriching direction temporarily increases when the incremental proportional term thus set to a larger value is applied, and then gradually increases due to application of an incremental integral term. However, thereafter, when the output from the exhaust gas concentration sensor has changed from the lean side to the rich side with respect to the predetermined reference value, the amount of change in the air-fuel ratio in the leaning direction temporarily increases due to application of the decremental proportional term, and then gradually increases due to application of a decremental integral term. Therefore, the resulting average air-fuel ratio cannot be shifted by a large amount in the enriching direction i.e to an air-fuel ratio much richer than the stoichiometric air-fuel ratio (A/F=14.7), at which the best conversion efficiency of the exhaust gas-purifying device is obtained. As a result the combustion temperature of the engine is so high as to cause deterioration of the catalyst of the exhaust gas-purifying device.
In order to eliminate this inconvenience, one will try to set the incremental proportional term to a still larger value to thereby increase the amount of change in the air-fuel ratio in the enriching direction. As a result, however, the amount of change in the fuel supply largely increases when the output from the exhaust gas concentration sensor has changed from the rich side to the lean side, which results in an increased amount of change in the output torque of the engine and hence degraded driveability of the vehicle.